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Subject[PATCH 13/17] cgroup_freezer: prepare for removal of TIF_FREEZE
Date
TIF_FREEZE will be removed soon and freezing() will directly test
whether any freezing condition is in effect. Make the following
changes in preparation.

* Rename cgroup_freezing_or_frozen() to cgroup_freezing() and make it
return bool.

* Make cgroup_freezing() access task_freezer() under rcu read lock
instead of task_lock(). This makes the state dereferencing racy
against task moving to another cgroup; however, it was already racy
without this change as ->state dereference wasn't synchronized.
This will be later dealt with using attach hooks.

* freezer->state is now set before trying to push tasks into the
target state.

-v2: Oleg pointed out that freeze_change_state() was setting
freeze->state incorrectly to CGROUP_FROZEN instead of
CGROUP_FREEZING. Fixed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Paul Menage <paul@paulmenage.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
---
include/linux/freezer.h | 6 +++---
kernel/cgroup_freezer.c | 36 ++++++++++++------------------------
kernel/power/process.c | 2 +-
3 files changed, 16 insertions(+), 28 deletions(-)
diff --git a/include/linux/freezer.h b/include/linux/freezer.h
index e6657778..243ec0b 100644
--- a/include/linux/freezer.h
+++ b/include/linux/freezer.h
@@ -63,11 +63,11 @@ static inline bool try_to_freeze(void)
extern bool freeze_task(struct task_struct *p, bool sig_only);

#ifdef CONFIG_CGROUP_FREEZER
-extern int cgroup_freezing_or_frozen(struct task_struct *task);
+extern bool cgroup_freezing(struct task_struct *task);
#else /* !CONFIG_CGROUP_FREEZER */
-static inline int cgroup_freezing_or_frozen(struct task_struct *task)
+static inline bool cgroup_freezing(struct task_struct *task)
{
- return 0;
+ return false;
}
#endif /* !CONFIG_CGROUP_FREEZER */

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 1a73727..2fae8b2 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -48,19 +48,17 @@ static inline struct freezer *task_freezer(struct task_struct *task)
struct freezer, css);
}

-static inline int __cgroup_freezing_or_frozen(struct task_struct *task)
+bool cgroup_freezing(struct task_struct *task)
{
- enum freezer_state state = task_freezer(task)->state;
- return (state == CGROUP_FREEZING) || (state == CGROUP_FROZEN);
-}
+ enum freezer_state state;
+ bool ret;

-int cgroup_freezing_or_frozen(struct task_struct *task)
-{
- int result;
- task_lock(task);
- result = __cgroup_freezing_or_frozen(task);
- task_unlock(task);
- return result;
+ rcu_read_lock();
+ state = task_freezer(task)->state;
+ ret = state == CGROUP_FREEZING || state == CGROUP_FROZEN;
+ rcu_read_unlock();
+
+ return ret;
}

/*
@@ -102,9 +100,6 @@ struct cgroup_subsys freezer_subsys;
* freezer_can_attach():
* cgroup_mutex (held by caller of can_attach)
*
- * cgroup_freezing_or_frozen():
- * task->alloc_lock (to get task's cgroup)
- *
* freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
* freezer->lock
* sighand->siglock (if the cgroup is freezing)
@@ -177,13 +172,7 @@ static int freezer_can_attach(struct cgroup_subsys *ss,

static int freezer_can_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
{
- rcu_read_lock();
- if (__cgroup_freezing_or_frozen(tsk)) {
- rcu_read_unlock();
- return -EBUSY;
- }
- rcu_read_unlock();
- return 0;
+ return cgroup_freezing(tsk) ? -EBUSY : 0;
}

static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
@@ -279,7 +268,6 @@ static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
struct task_struct *task;
unsigned int num_cant_freeze_now = 0;

- freezer->state = CGROUP_FREEZING;
cgroup_iter_start(cgroup, &it);
while ((task = cgroup_iter_next(cgroup, &it))) {
if (!freeze_task(task, true))
@@ -303,8 +291,6 @@ static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
while ((task = cgroup_iter_next(cgroup, &it)))
__thaw_task(task);
cgroup_iter_end(cgroup, &it);
-
- freezer->state = CGROUP_THAWED;
}

static int freezer_change_state(struct cgroup *cgroup,
@@ -323,9 +309,11 @@ static int freezer_change_state(struct cgroup *cgroup,

switch (goal_state) {
case CGROUP_THAWED:
+ freezer->state = CGROUP_THAWED;
unfreeze_cgroup(cgroup, freezer);
break;
case CGROUP_FROZEN:
+ freezer->state = CGROUP_FREEZING;
retval = try_to_freeze_cgroup(cgroup, freezer);
break;
default:
diff --git a/kernel/power/process.c b/kernel/power/process.c
index 85ecaec..54ba145 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -166,7 +166,7 @@ void thaw_processes(void)

read_lock(&tasklist_lock);
do_each_thread(g, p) {
- if (cgroup_freezing_or_frozen(p))
+ if (cgroup_freezing(p))
continue;

__thaw_task(p);
--
1.7.3.1


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